Improve file change tracking

[dgageot]

Adding some tests to the code that detects file changes.
Also simplifies the code used to track changes.

This should make the watch mode consume less CPU.
I'll run some tests before getting this PR merged to know how much

[codecov-io]

Codecov Report

Merging #888 into master will decrease coverage by 0.11%.
The diff coverage is 77.27%.

@@            Coverage Diff             @@
##           master     #888      +/-   ##
==========================================
- Coverage   38.38%   38.27%   -0.12%     
==========================================
  Files          54       56       +2     
  Lines        2519     2576      +57     
==========================================
+ Hits          967      986      +19     
- Misses       1441     1476      +35     
- Partials      111      114       +3

Impacted Files	Coverage Δ
pkg/skaffold/watch/changes.go	76% <77.27%> (ø)
pkg/skaffold/watch/watch.go	0% <0%> (ø)

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[balopat]

@dgageot, the new tests seem to be failing, can you check?

[dgageot]

@balopat Works on my machine. lol

[dgageot]

@balopat should be good now. Funny to see the coverage decrease when I added tests and removed some code.